This invention relates to the art of materials science and, more particularly, to nonmetallic materials and powder metallurgy.
Ceramic materials have certain outstanding properties, such as high temperature strength, corrosion resistance, low density, and low thermal expansion, which make them attractive materials for high temperature applications. However, ceramics differ from metals in one very important aspect; they do not show any yield upon loading. The lack of a stress-relieving characteristic, which gives ceramics their brittle nature and low tolerance for flaws, is a major drawback to using them in high-temperature structural applications.
There is a class of materials which offers the advantages of a ceramic and certain of the beneficial mechanical characteristics of a metal. These materials are intermetallics, which at high temperature have the excellent properties of a ceramic, but mechanically behave more like a metal, since they show yielding and stress-relieving characteristics.
Molybdenum disilicide (MoSi.sub.2) is an intermetallic compound which has potential for structural use in oxidizing environments above 1200.degree. C. It has a melting point of 2030.degree. C. and its oxidation resistance at high temperature is very good. Mechanically, MoSi.sub.2 behaves as a metal at high temperatures; it undergoes a brittle-to-ductile transition at approximately 1000.degree. C. Thus, MoSi.sub.2 has a stress-relieving characteristic at high temperatures. The major problems impeding the use of MoSi.sub.2 as a high temperature structural material with potential use temperatures in the range of 1200.degree.-1800.degree. C. are its relatively low strength at high temperatures and its brittleness, which may be referred to as lack of fracture toughness, at low temperatures. Fracture toughness may be defined as resistance to fracture. At low temperatures, strength is limited by brittle fracture, while at high temperatures, it is limited by plastic deformation or creep. For this material to be a viable structural material at high temperatures, both its elevated temperature strength and its room temperature fracture toughness must be improved. The present invention addresses the problem of high temperature strength, though improvement in low temperature strength and fracture toughness may also be realized.
Silicon carbide whiskers made by a vapor-liquid-solid (VLS) process have been used to reinforce MoSi.sub.2 by means of dispersion strengthening mechanisms. This resulted in improved ambient temperature fracture toughness and a near doubling of strength at 1200.degree. C. compared to room temperature strength. The use of silicon carbide whiskers made by a vapor-solid (VS) process as a reinforcing material provided an improvement over VLS whiskers at high temperatures, but further improvement in strength at high temperatures is needed. This improvement may be attained by replacing a portion of the MoSi.sub.2 matrix with one or more refractory metal silicides. The refractory silicide will provide a solid solution strengthening effect or precipitation strengthening effect. Also, there are advantages in using SiC in powder form as the reinforcing material.